The proposed research is planned to utilize the unique opportunities for genetic studies of tumorigenesis provided by the CHEF cell line of Chinese hamster embryo fibroblasts which we have developed. (1) Because the CHEF/18-1 cell line is non-tumorigenic in nude mice and anchorage-dependent for growth, we can assay quantitatively for transformation and tumorigenesis after treatment of cells with mutagens, carcinogens, and promotors. We plan to use cloning in methyl cellulose to score transformation, and our coinjection technique to assess tumor forming ability quantitatively in the nude mouse. (2) The transformed and tumor forming clones recovered in induction experiments as well as other classes of mutants isolated in the same experiments, will be used for genetic mapping. Identification of chromosomes carrying genes that influence tumorigenicity will be carried out with hybrids that have undergone chromosome reduction. Further localization will be attempted with gene transfer methods: using microcells, isolated metaphase chromosomes, and DNA restriction fragments. (3) A detailed cytogenetic study of the mechanism of chromosome loss is planned, in view of the remarkable reduction in chromosome number from tetraploid to the diploid range that occurs in selected cell hybrids. As soon as linked markers become available, we will attempt to induce genetic recombination during chromosome reduction in cell hybrids.